Channeling and microactivation of materials

Nuclear Instruments and Methods in Physics Research B40/41 North-Holland, Amsterdam

CHANNELING

AND MICROACI’IVATION

OF MATERIALS

C.J. MAGGIORE, J.D. BLACIC, G. BLONDIAUX *, J.L. DEBRUN E. MATHEZ +, M.A. MISDAQ ++ and M. VALLADON * Cenier for Materials

1193

(1989) 1193-1195

Science, Los Alamos National Laboratory,

Los Alamos,

*, M. HAGE AL1 * *,

NM 87545, USA

Charged particle activation analysis can be combined with channeling to determine lattice location of impurities at the trace level in single crystal samples. It can also be used with a nuclear microprobe to measure impurities at trace levels in small or spatially inhomogeneous samples. Examples of these extensions of activation analysis to realistic samples are carbon determination in organometallic vapor phase epitaxial layers of GaAlAs and GaAs and oxygen determination in diamonds.

1. Introduction Charged particle sidered

termination. produce

activation

to be a bulk technique Low

isotopes

energy suitable

(few

analysis

is usually

for trace

element

MeV) accelerators

for the direct

condecan

determination

of light elements such as B, C and 0 in a wide variety of

materials, and they have been used to calibrate standards for other analytical methods such as secondary ion mass spectroscopy. However, many questions of interest involving trace elements concern the location of the element, either within the crystal lattice or within a small region of an inhomogenous sample. An example of the first case is the role of light element impurities in semiconductors. The electrical properties of dopants depend on both the concentration and location of the dopant species in the host lattice. Ion channeling using Rutherford backscattering or prompt nuclear reaction analysis has been widely used to study the location and role of dopants and impurities in single crystals, but the concentration is usually greater than 0.1% even in favorable cases [l]. Highly doped semiconductors could be used and the results extrapolated to trace levels, but this is not always warranted. Charged particle activation has the required sensitivity to measure carbon in GaAs to levels well below 1 ppm and we have combined channeling with activation to study it in epitaxial layers of undoped GaAlAs on GaAs.

* CNRS, Centre D’Etudes et de Recherches Par Irradiation, Orleans, France. ** CNRS, Centre de Recherches Nucleaires, Strasbourg, France. + American Museum of Natural History, New York, NY, USA. ++ University of Rabat, Morocco.

0168-583X/89/$03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

Another example of a problem requiring the combination of activation with a method of localization is the study of volatile trace elements in diamonds. Trace element studies in natural geologic specimens usually require spatial localization to measure the small and sometimes zoned samples. The study of oxygen in diamonds is interesting because the absolute trace concentration and possible zoning in naturally occurring specimens are measures of the conditions existing at the time of formation. The concentration range of interest is less than a few hundred ppm. Charged particle activation analysis is able to detect oxygen in a carbon matrix at these levels and using a nuclear microprobe to focus the beam on the sample can provide the required spatial localization.

2. Channeled activation Epitaxial layers of GaAlAs 25 pm thick were grown on GaAs substrates by vapor phase epitaxy. The Al content of films was controlled by varying the ratio of trimethylaluminum (Al(CH,),) to trimethylgallium (Ga(CH,),) and the growth rate was controlled by the arsine (ASH,) flow rate. Films with aluminum contents of 52X, 32% and 25% relative to Ga were grown. A 2.1 MeV deuteron beam from the 4 MV Van de Graaff accelerator of the CNRS at Strasbourg, France was used to produce the positron emitter 13N by the reaction “C(d, n)13N. Random and channeled irradiations along the (100) and (110) axes were performed and the i3N yield was measured from the 511 keV annihilation radiation with a Ge(Li) detector. The 15 min irradiations were performed with approximately 1 l.tA of beam, and a 2 urn thick surface layer was removed after activation and before counting. This latter step is necessary to remove the activated carbon layer that is deposited during irradiation [2]. X. ACTIVATION

ANALYSIS

C.J. Maggiore et al. / Channeling and microactivation

1194

The absolute carbon content was determined by comparing the normalized 13N yield from the sample activated in a random direction to the normalized yield from a graphite standard. Corrections for the sample etching were made and the stopping power data of Andersen and Ziegler [3] were used with the average stopping power method [4]. The bulk carbon content of samples varied between 0.3 and 2.8 ppm by weight. Two general trends in the total carbon content were observed; the total carbon increased with Al content and the carbon decreased with growth rate 151. By comparing the angular yield curves along various low index directions it is possible to determine the average site of the impurity within the crystal lattice, and in a normal channeling experiment using the backscattering signal it is relatively straightforward to acquire such data. It is much more difficult to obtain this complete set of data when the activation yield is the signal. Therefore we compared the yields in the (100) and (110) axial directions to the random yield. The ratio of the channeled yield to the random yield (C/R) along the (100) direction was not constant with either carbon content, growth rate, or Al content. If the carbon were only present in a single site, we would not expect this complex behavior. The ratio C/R was less than 1 for carbon content greater than 1 ppm, and C/R was greater than 1 for total carbon less than 1 ppm. This is only consistent with carbon occupying the interstitial tetrahedral site at low concentrations and a substitutional

or octahedral

interstitial

site at high con-

centrations. The C/R ratio in the (110) direction also showed complex behavior with growth conditions. At high carbon concentration (> 1.7 ppm) C/R is also less than 1, consistent with the (100) data, and implying the carbon is primarily in a substitutional site. Fig. 1 shows the data for the C/R ratios in the two axial directions for samples grown under the same conditions. A more

F Mixed Site

CT G

1 .O -

0

Substitutional Site C>1.7ppm l

0.0 I . ’ . - 1 . ’ 0.0 0.5 C/R

. ’ ’ . . ’ ’ ’ ’ . ’ 1.0

1.5

2.0



Fig. 1. The ratio of channeled to random activity (C/R) (100) direction vs C/R in the (110) direction.

in the

of materials

complete description of the data and its interpretation has been given elsewhere [5], but the main conclusions are that carbon initially occupies the interstitial tetrahedral site and then fills the As vacancies. It is not possible to see the effects observed here with samples that have been doped to the relatively high concentrations needed for prompt analysis. The inability to get full angular scan yields in a reasonable time means that channeled activation cannot be used to determine precise localization within a symmetry region. Such determinations require a comparison of the experimental data to calculations of the angular scan. Two other effects limit the applicability of channeled activation, radiation damage to the lattice during alignment and irradiation and surface contamination. The carbon contamination on the surface must be removed before the sample can be counted; the 2 pm surface etch is actually removing the most highly activated part of the sample and the most effectively channeled. Performing the experiment completely in ultrahigh vacuum could eliminate the etching step entirely.

3. Microactivation Charged particle activation is a bulk technique confined to the outer several microns of the sample and averaged over the dimensions of the incident beam. The spatial resolution is determined by the area of the incident beam, the depth of excitation in the sample and the beam spreading in the sample. For the analysis of oxygen in diamonds we used the 16q3He, p)‘*F nuclear reaction. The “F decays by positron emission with a half-life of 109.6 min. In diamonds there is a competing reaction induced by the 3He beam, the ‘* C( 3He, 4 He)” C reaction, which produces the positron emitter “C with a 20.3 min half-life. By waiting for the shorter-lived “C activity to decay, the trace activity from the l*F may be measured to determine the oxygen in the original diamond. Sensitivity for the detection of oxygen in diamond by activation with a nuclear microprobe is determined by the total integrated charge that can be focussed into a defined volume of the sample in a given time. We have demonstrated both sensitivity to oxygen in diamond using the ( 3He, p) reaction and the abilty to make spatially resolved measurements with the Los Alamos nuclear microprobe. We focussed a beam of 4 MeV 3He particles on a type IIA diamond 1 X 1 X 25 mm3 from Drukker. An average beam current of 6.8 nA was used to activate the sample for 110 min. The resultant positron activity was counted in a gammagamma coincidence measurement using two sodium iodide detectors. Fig. 2 shows the annihilation activity versus time after waiting 34 h for most of the “C

C.J. Maggiore et al. / Channeling and microactivation

1195

of materials

damage in the sample. Improved spatial resolution can be obtained with some loss in sensitivity if necessary. The case presented here of oxygen determination in a carbon matrix is in many ways a pathological one. Much better sensitivity is possible for oxygen in samples without carbon.

4. Conclusion

0 0

15

30

45

60

75

90

TIME(iOOOSEC)

Fig. 2. 511 keV annihilation activity vs time for a diamond activated with a 4 MeV 3He microbeam.

activity

to decay. The data were fit using using the sum

of two exponentials decay

constants

with a constant

background.

for the two exponential

terms

The corre-

spond to the 20.3 and 109.6 min half-lives of the “C and ‘*F. There was no evidence of other positron activity with any other decay constant. The sensitivity of the activation and detector efficiency was calibrated using an alumina standard. Correcting for the known difference in stopping power between diamond and alumina yielded an atomic concentration of 83 & 7 ppm. The uncertainty is determined by the counting statistics, the accuracy of the current integration, and the stopping power correction. The ultimate sensitivity of the activation method described here is set by the background counting rate in the detector. Using the existing detection system we estimate a detection limit of < 40 ppm. The feasibility experiment described here was not optimized in terms of spatial resolution, but calculations based on the known characteristics of the microprobe indicate that this detection limit is readily obtained with better than 50 urn resolution. We consider this to be a very conservative estimate based on a lack of experience with 3He beams in the microprobe and possible beam induced

The strength of charged particle activation analysis is that it is more sensitive than prompt nuclear reaction analysis for many light isotopes. There are difficult analytic problems involving the lattice location of light impurities at trace levels that can be successfully solved by combining the enhanced sensitivity of activation with the site specificity of channeling. Activation can also be combined with a nuclear microprobe to provide localized quantitative microanalysis of certain light elements in spatially inhomogeneous samples such as geologic materials. The sensitivity using these combined methods is less than normal bulk analysis, but better than prompt nuclear reaction analysis. The authors wish to thank Mark Hollander, Caleb Evans and Joe Tesmer of the Los Alamos Ion Beam Materials Laboratory for their help with the microprobe aspects of the work. This work was supported in part by the US Department of Energy.

References

PI L.C. Feldman, J.W. Mayer and S.T. Picraux, Materials

Analysis by Ion Channeling (Academic Press, New York, 1982). PI G. Blondiaux, C.S. Sastri, M. Valladon and J.L. Debrun, J. Radioanal. Chem. 56 (1980) 163. 131 H.H. Andersen and J.F. Ziegler, Hydrogen Stopping Powers and Ranges in all Elements (Pergamon, New York, 1977). 141 K. I&ii, M. Valladon and J.L. Debrun, Nucl. Instr. and Meth. 150 (1978) 213. [51 M.A. Misdaq, G. Blondiaux, J.P. Andre, M. Hage Ali, M. Valladon, C.J. Maggiore and J.L. Debrun, Nucl. Instr. and Meth. B15 (1986) 328.

X. ACTIVATION

ANALYSIS